Evaporites-form when sea water evaporates; often at divergent margins as the process is just changing from continental rifting to seafloor spreading. In the image below, you can see a typical evaporite. It doesn't look like table salt yet, because it has red clay and black organic matter in it.
With the question of dolomite formation, we have entered the evaporite
environment. Within continental margins, the bulk of hydrogenous sediments
are evaporitic salt. Strictly speaking, calcareous shells and skeletons
are also hydrogenous, since they originate in the water. However, we have
called the minerals precipitated by organisms biogenous, and set them
apart.
3.8.1 Marine Evaporites are those sediments
which form on evaporation of seawater. Restriction of exchange with the open ocean, in a semi?enclosed basin, is necessary to drive the salt content high enough for precipitation to begin. Such restricted bodies of water are (1) coastal lagoons; (2) salt seas on the shelves; or (3) early rift oceans in the deep sea. A special case is the Mediterranean, which was partially isolated in the latest Miocene, 6 to 5 million years ago (see Sect. 9.5.2).
How much salt can be produced by evaporating a 1000m high column of sea water? Salt constitutes 3.5 % (or 35 0/00) of the weight of the column, its density is about 2.5 times that of water. Thus, we would obtain about 14 m salt. Most of this would be table salt (halite) (see Table 3.1). The least soluble salts precipitate first: calcium carbonate (aragonite), and calcium sulfate (gypsum). To precipitate halite, the brine needs to be concentrated about tenfold. Many evaporites only contain carbonate and gypsum (or anhydrite), others have thick deposits of halite or, rarely, of the valuable (and very soluble) potassium salts.
This sequence of mineral precipitation was experimentally established
by Usiglio in 1849.
An evaporite basin, besides having restricted access to the open ocean,
must fie within an arid climate. New seawater must be delivered from time
to time. This seawater must be concentrated by evaporation. If only gypsum
is to form, a concentration beyond threefold must be prevented, by new
incursions of seawater, or else halite must be removed during or after
each evaporation cycle (Fig. 3.11). If only halite is to form, left over
brine from a gypsum?precipitating basin must be available, or the halite
must be leached from elsewhere and brought into the basin by nonmarine
waters. Differential preservation, then, and serial fractionation are the
key processes in controlling the chemistry of salt deposits. In lagoonal
settings, a periodic covering of existing salts, by wind or flood
deposits, may be necessary to prevent redissolution by the next invasion
of ocean water.
Only very few modern examples for submarine evaporite formation are
known: coastal lagoons of Ojo de Liebre in Baja California (Mexico) or the
20km long drowned river valley Bocana de Virrila (Peru), where gypsum is
precipitated when salinities reach 160 0/00 by
evaporation and halite at more than 320 0/00 (type a
in Fig. 3.11).
The formation of diagenetic dolomite and gypsum/anhydrite in coastal
"sabkha" environments was discussed in the last section.
Features typical for this environment, such as big gypsum crystals,
nodular or intensely deformed anhydrite, and algal mats, are well known in
ancient evaporite series of the Permian period (Lower Clear Formation,
Texas; Zechstein, Northwestern Europe). Also, such features were
discovered in uppermost Miocene sediments, underlying the Mediterranean,
by deep-sea drilling (see Fig. 3.12).
Text and figures are all from Seibold and Berger (1993): The Sea Floor. An
introduction to marine geology. Second Edition, Spinger Verlag, pages
88-92, figures 3.10 to 3.12. All rights with Fischer Verlag and the
authors of this book.
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